what you can do when exporting e.g. "AVI" files is setting the "FrameRate". When you want to have for instance every frame displayed for 1 second the frame rate is 1 frame/second too. Unfortunately, this does only work for integer values and to have an image displayed for s seconds, you would have to set frame rate to 1/s.
One bad hack is to set the frame rate to 1 and use several copies of the same frame. A better option here might be to use external tools. See for instance the link Jens gave in the comments

You've got a typo: it's FrameDurations, not FrameDuration. Without the s at the end, your exported movie doesn't actually have the duration you specified, but instead uses the default.
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JensJan 11 '13 at 6:49

@Jens Thanks, I corrected my type (without using your Typo ;-) and now it works as expected.
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halirutanJan 11 '13 at 7:33

This takes a list of frames, e.g. the ones in halirutan's answer, and exports them as a movie in AVI or MOV or similar formats. These formats recognize the "FrameRate" option in Export, and you can calculate the desired frame rate as the reciprocal of the frame duration in seconds.

In this function, I generalized this to a variable frame rate by allowing an optional third argument that contains a list of individual frame durations. If the list is shorter than the number of frames, it's repeated. To make certain frames in the list of the first argument appear longer, I simply replicate them a number of times depending on how much longer than the minimum duration in the list of the third argument is.

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